There's no right or wrong answer to this. One species inhabits brackish water more often than freshwater, Datnioides campbelli. Another species is more typical of freshwater than brackish water though it is certainly found in both, Datnioides polota (= quadrifasciatus). Yet others appear to be confined to freshwater and are not commonly found in brackish water, if ever, i.e., Datnioides undecimradiatus, Datnioides microlepis, and Datnioides pulcher.
Even the freshwater species are fairly tolerant of brackish water, so you could probably keep them in a low-end brackish tank (SG 1.005 or less) without any real problems. As a family these fish are very closely related to the Lobotidae and are definitely classed as secondary freshwater fish. Because of the potential difficulties in identifying juveniles it may even be sensible to keep them in brackish water until they are convincingly identified. (It should be added that while many fishkeepers think they can identify the species, ichthylogists working on the group are much less complacent; see Roberts and Kottelat 1994 and Kottelat, 1998. There's an excellent summary in the Aqualog brackish water fishes book.)
The two brackish water species can be kept in either low or moderately brackish water, though I'd recommend around 1.003-1.006 as being adequate and providing flexibility for adding plants and other fishes. I'm not aware of anyone keeping Datnioides campbelli at a marine salinity, though it may be possible given their ecological range.
Cheers, Neale